The present invention relates to a storage unit system for performing duplexing of data (backup of data) between a primary side control unit connected to a processing unit (host computer) and a secondary side control unit connected to the primary side control unit.
As a method for carrying out duplexing a data between primary side and secondary side control units, a method is generally known, by which primary side and secondary side control units are connected via an interface cable, and the primary side control unit issues a write request received from a processing unit to a secondary side control unit, and duplexing of data is carried out.
Further, a method for duplexing of data is known, in which a primary side system and a secondary side system each provided with a plurality (“M” and “N” in numbers) of control units respectively and a write request is issued from the processing unit to “M” primary side control units. This method is disclosed in J-P-A-No. 85408/1999.
Where the primary side system and the secondary side system each has a plurality (“M” and “N” in numbers) of control units, each of the primary side control units adds a write time indicating time of issuance of the write request to the write request received from the processing unit (host), and issues a write request to each of the secondary side control units connected via the interface cable. Then, the write data and the write time are transmitted to the secondary side control units. On the other hand, in the secondary side system, one of the plural secondary side control units is designated as a master control unit, and the designated master control unit and the other secondary side control unit are connected on one-to-one basis via data transfer route. The secondary side control units other than the master control unit transmit information relating to the write time (write time from the host to the primary side control unit) of the write data received from the primary side control unit to the master control unit on the secondary side. The master control unit refers to the information relating to the write information received from all of the secondary side control units (including the write time information of the master control unit itself) and determines guarantee time. Here, guarantee time is defined as “a time, which can guarantee that all of the write data with the write time older than guarantee time (write time from the host to the primary side) on all of the secondary side control units have been received at all of the secondary side control units”. Specifically, the determined guarantee time guarantees that all write data relating to all write requests issued from the processing unit to each of the primary side control units before the guarantee time have been received (already duplexed) at the secondary side control units. The master control unit notifies the determined guarantee time to all of the secondary side control units, and each of the secondary side control units can confirm that duplexing is guaranteed for the write data, which have write time older than the guarantee time. When this guarantee time is notified, even in case the data on the primary side is destroyed by some accident, for instance, the data from the primary side control unit received before the guarantee time can be utilized.